The present invention relates to an electric power system of an alternating current or a direct current, and in particular, to an electric power system which levels the magnitude of a load as viewed from a power source, when the magnitude of the load varies with time.
In a prior art system for leveling a load of a system in which the magnitude of the load with respect a power source varies, as described in JP-A-9-182316, a distributed electric power system is known in which a power converter linked to a power system is provided, and when the load is small at night or the like, the power is accumulated or stored in power accumulating or storing means through the power converter, and when the load is large as in a peak load time in the daytime, the power accumulated in the power accumulating means is discharged. Also, in case of power failure in the electric power system, the power stored in the power accumulating means is supplied to the load through the power converter, thereby to continue the operation of the load even at the time of power failure.
In such a distributed electric power system, the power supplied to the load is monitored, the magnitude of the power output to the power system in response to the magnitude of the load power is calculated, and the power is discharged in accordance with the calculated value from the power accumulating means. On the other hand, when the value of the load power during the monitor is small, the magnitude of residual power viewed from the power system is calculated in the same way, and the power is accumulated in accordance with its value.
In the system as mentioned above, the magnitude of the power at the load side is detected, monitored, and the power to be discharged or charged is determined depending on its value. As a result, when the power change is relatively slow, as in the case of leveling the change in the power in the daytime and at night, it is possible to control the discharged power sufficiently fast. However, when the magnitude of the load is changed frequently and rapidly depending on the use condition of the equipment, for example, as at the time of starting the load in household, such as a microwave oven or an electromagnetic cooking device or air-conditioner, if the discharging power is calculated after detecting the load power, the power matched with the change of the load can not be supplied accurately. Thus, it is necessary to supply the power to the load from the power system by the deviated amount of the discharged power.
Furthermore, the discharged power from the power accumulating means is output to the power system through the power converter. For this reason, the accuracy of the discharged power is degraded due to the influence of loss of the power converter. Here, the discharged power of the power accumulating means depends not only on the discharged current but also on the voltage of the power accumulating means. Generally, its voltage is reduced due to the discharge from the power accumulating means. These are the error factors of the discharged power control. In this manner, due to the control error of the power accumulating means or the power converter, a deviation occurs between the discharged power from the power accumulating means and the load power which is desired to level off, and the peak of the load power can not be suppressed sufficiently. Furthermore, when the discharged power from the power accumulating means has become larger than the load power, there is a problem of reverse flow of the discharged power into the power system.
By making the load power level off, there is no need to prepare a power source capacity of the power system matched with the peak load value, and it is possible to have the power source capacity matched with the leveled power. However, in such a conventional technique, because of the control lag and error of the discharged power, it is necessary that the power source side be able to supply a peak power corresponding to the amount of error, and it is impossible to reduce the power source capacity.
The object of the present invention is to provide an electric power system in which with respect to a load power which varies rapidly, power is discharged from the power accumulating means to suppress the power peak, and the power source capacity is reduced to a value matched with a leveled capacity.
In order to achieve the above-mentioned object, an electric power system according to one aspect of the present invention, comprises a power source, a load supplied with power from the power source, a battery for discharging power to the load and for accumulating or storing power from the power source, a power detector for detecting the power supplied from the power source, a power converter for power converting the power charged to and discharged from the battery, and a power controller for controlling the power which is power converted by the power converter, and wherein when a detection value of the power supplied to the load from the power source exceeds a predetermined power upper limit value, the power discharged to the load from the battery is controlled on the basis of a deviation between the detection value of the power and the power upper Limit value.
Furthermore, an electric power system according to another aspect of the present invention, comprises a power source, a load supplied with power from the power source, a battery for discharging power to the load and for accumulating power from the power source, a power detector for detecting the power supplied from the power source, a direct current power converter for controlling the charging and discharging of the battery, a further power converter for controlling the output voltage of the direct current power converter at constant irrespective of the charge and discharge of the battery, and a power controller for controlling the power charged and discharged by the battery, and wherein, when a detection value of the power supplied to the load from the power source exceeds a predetermined power upper limit value, the power discharged to the load from the battery is controlled on the basis of a deviation between the detection value of the power and the power upper limit value.
In one embodiment, when a detection value of the power supplied to the load from the power source is equal to or lower than a predetermined threshold value, charging power is supplied to the battery.